It takes like 5 good swaps in less than 1 minute for you to be able to kill them. It goes a bit like this:
First you have to go through barkskin, then ironbark, then brocolli. Then you have to go through barkskin and ironbark again, but it's ok this time he has no brocolli (GG!). You then need to force his trinket while 2nd barkskin and ironbark are both on CD, and odds are you might think this is it! However after forcing his trinket, and being in between the 40 second cycle which is in another 3 min cycle (cause of brocolli) you still can't silence him because he'll just displacer beast away.
tl;dr Resto Druids are only viable kill targets for 10 seconds each 2 minutes - and you probably need a dr'ed stun for it.